The present invention relates to a process for producing a strengthened fiber assembly containing cellulosic fibers with the cellulose being present in the crystalline structure of cellulose II, by contacting the fiber assembly with an aqueous solution of N-methylmorpholine-N-oxide (NMMO) at elevated temperature and subsequently washing the fiber assembly.
For the purposes of this description and the annexed claims, the expression "fiber assembly" is to denote any kind of wovens, nonwovens or random webs.
During the past years, a number of processes in which cellulose is dissolved in an organic solvent, a combination of an organic solvent with an inorganic salt, or in aqueous salt solutions, without forming a derivative have been described as alternatives to the viscose process. Cellulose fibers made of such solutions were given the generic name lyocell by BISFA (The International Bureau for the Standardisation of Man-Made Fibres). According to the BISFA definition, lyocell is a cellulose fiber obtained from an organic solvent by means of a spinning process. By "organic solvent", a mixture of an organic chemical and water is understood by BISFA.
To date but a single process for the production of a cellulose fiber of the lyocell type has, however, become accepted to the point of large-scale realization, namely the amine oxide process. In that process, N-methylmorpholine-N-oxide (NMMO) is preferably used as the solvent. For the purposes of the instant specification, the term "tertiary amine oxides" substitutionally is referred to by the abbreviation "NMMO", NMMO additionally representing the presently preferred N-methylmorpholine-N-oxide.
Tertiary amine oxides have been known for long as alternative solvents for cellulose. Thus, it is known, for instance, from U.S. Pat. No. 2,179,181 that tertiary amine oxides are capable of dissolving high-quality chemical pulp without the formation of derivatives and that cellulose moulded bodies such as fibers may be obtained from such solutions by precipitation. U.S. Pat. Nos. 3,447,939, 3,447,956 and 3,508,941 describe further processes for the production of cellulose solutions with cyclic amine oxides being used as the preferred solvents. In all of those processes, cellulose is physically dissolved at elevated temperature.
In EP-A - 0 356 419 to applicant a process is decribed, which preferably is carried out in a thin-layer treating apparatus in which a suspension of the comminuted pulp in an aqueous tertiary amine oxide is spread in the form of a thin layer, transported over a heating surface, while the surface of the thin layer is exposed to a vacuum. During transportation of the suspension over the heating surface water is evaporated and the cellulose can be dissolved such that a spinnable cellulose solution is discharged from the thin-layer treating apparatus.
A process and an arrangement for spinning cellulose solutions are known, for instance, from WO 93/19230 to applicant. There, the spinning solution is spun in the hot state and the filaments obtained are introduced into a precipitation bath in order to precipitate the cellulose contained therein, the filaments being cooled prior to their introduction into the precipitation bath. Cooling is effected immediately after moulding and, preferably, consists in horizontally blowing air at the cellulose moulded body.
German Patent No. 902 427 describes the strengthening of a fleece of cellulose fibers by means of a lye bath containing 5 to 15% NaOH. The celluose fibers are swollen by the lye bath and thereby strengthened.
From WO 95/07386 to applicant, a process for producing paper is known, in which an aqueous suspension of commninuted cellulose material is mechanically treated and subsequently subjected to a sheet forming procedure, the suspension containing tertiary amine oxide. That process allows for the production of high-strength paper.
A process for strengthening a fibrous material is known from U.S. Pat. No. 3,447,956. Strengthening is effected in that the fibrous material is soaked with an amine oxide and heated to a temperature at which the amine oxide is able to strengthen the fibrous material. Proposed fibrous materials are wovens and nonwovens containing natural cellulosic fibers such as, e.g., wood pulp, cotton, linen, but also synthetic fibers such as rayon (viscose fibers). Particularly preferred is the treatment of paper with amine oxide. In doing so, NMMO apparently is used as a monohydrate in the molten or liquid state or dissolved in a volatile solvent capable of being evaporated.
From WO 96/37653 fiber assemblies provided with a cellulosic coat, impregnation or sheath are known. Those fiber assemblies are produced by coating the fiber assembly on one side with a solution of cellulose in aqueous NMMO, whereupon the layer is coagulated in a water bath.
U.S. Pat. No. 4,196,282 describes the three-component system NMMO/H.sub.2 O/cellulose and the so-called "dissolution frame", i.e., those conditions under which the cellulose dissolves in aqueous NMMO.
Furthermore, it is known that cellulose fibers may be swollen with NMMO (Chancy et al., "Swelling and Dissolution of Cellulose in Amine Oxide/Water Systems"; J. App. Pol. Sci: Appl. Pol. Symp. 37,239-259 (1983)).